I've hired from Rome Ciampino airport twice before. Avis, enterprise, dollar, etc, check the rates. Wasn't too bad a price 6 or 7 years ago. Be careful you photograph condition and any blemishes, but saying that I didn't have any problems.
Traffic on Ring Road was surprisingly OK, much less than M25. Didn't drive into Rome itself as visited my sister who lives 40 miles north of Rome, but provided you have a sat nav and reliable Co driver it should be OK.
Enjoy.